Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Strategies have been developed for prediction and/or early detection of pre-ignition based on engine operating conditions. Additionally, following detection, various pre-ignition mitigating steps may be taken.
One example approach for addressing pre-ignition is shown by Brehob in U.S. Pat. No. 7,178,503. Therein, an injection timing of a direct injector is adjusted to reduce pre-ignition. In particular, the injection timing is adjusted so as to center the injection duration roughly at BDC. Alternatively, the injection timing is adjusted to spray the fuel onto the piston (e.g., early in intake stroke or late in compression stroke).
However, the inventors herein have identified potential issues with such an approach. As one example, different cylinders may have differing propensities for pre-ignition. Thus, the same injection timing adjustment may reduce pre-ignition in one cylinder but not another. As another example, even with the adjusted injection timing, pre-ignition may not be sufficiently reduced. Further still, the injection timing adjustment may lead to undesirable torque disturbances between the cylinders.
Thus in one example, at least some of the above issues may be partly addressed by a method of operating an engine comprising, adjusting a timing and number of injections of a fuel injection to an engine cylinder, in a single engine cycle, based on a pre-ignition count of the cylinder.
In one example, an engine control system may compare the pre-ignition counts of each engine cylinder. A first engine cylinder with a higher pre-ignition count may be rich injected while a second cylinder with a lower pre-ignition count may be lean injected. Further, the first cylinder may be operated in a split injection mode such that the rich fuel injection is delivered as multiple injections in the same engine cycle. In comparison, the second cylinder may be operated in a single injection mode such that the lean fuel injection is delivered in a single injection.
The number of injections as well as the timing of each injection to the first cylinder may be based on the pre-ignition count of the first cylinder. Likewise, the timing of the lean fuel injection may be based on the pre-ignition count of the second cylinder. As one example, as the pre-ignition count of the first cylinder increases, the number of injections may be increased. As another example, the injection timing of the multiple injections in the first cylinder may be adjusted to either advance or retard the average injection timing from TDC to place the rich injection timing in a pre-ignition reducing injection timing (or timing range). The injection timing adjustment may depend on the amount of fuel injected in each of the multiple injections (that is, a split ratio of the injections) as well as the pre-ignition count of the first cylinder.
In this way, by adjusting the fueling of each cylinder of an engine based on respective pre-ignition counts, engine pre-ignition may be reduced even if different cylinders of the engine have different likelihoods of pre-ignition. Further, by using a split rich injection in cylinders with a higher propensity for pre-ignition, the pre-ignition mitigating effect of the enrichment may be improved. In this way, engine degradation due to pre-ignition can be reduced while improving engine fuel economy and exhaust emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.